1. Field of the Invention
The invention relates to a planar antenna, in particular for mobile radio, the planar antenna having two conductive layers arranged at a predefined distance from one another.
2. Background of the Invention
The application area of the invention relates primarily to the mobile radio field and, in this case, in particular to E and D networks.
Known antenna solutions for the field of mobile radio applications are based on linear antenna designs in the form of monopole arrangements in shortened or unshortened embodiments. These linear antennas are known both as externally mountable onboard antennas and components directly coupled to the terminal, and are subject to different directionality and efficiency, these components being exclusively omnidirectional in the azimuthal plane. Known patch antenna solutions are based on dipole-like configurations which are arranged in two-dimensional fashion and whose directional diagrams is irregular and, in connection with the respective background, exhibit the features of significant radiation field deformation. The radiation properties relating to the application field are significantly inferior to those of conventional linear antennas. Likewise, controlled masking properties of the radiation diagram are not demonstrable. No further solutions are known whose electromagnetic or radiation properties are obtained on the basis of asymmetric and open waveguide technology, in particular microstrip technology, with the use of self-supporting conductive sheet conductors or sheet-like conductor surfaces.
EP 0 176 311 discloses a planar antenna which has a ground plane which is kept at a distance from the radiator element by means of a dielectric substrate layer. The radiator element is fed by means of a coaxial waveguide and is electrically conductively connected by means of short-circuit connections on one side to the ground surface. The radiator element is a geometrical subregion of the ground plane. EP 0 176 311 also discloses a two-dimensional short-circuit connection between the ground plane and the radiator element.
DE 195 04 577 discloses a mobile radio antenna for motor vehicles, which also has a radiation element that is in connection with the inner conductor of a coaxial waveguide. One side of the radiator element is in conductive connection with a ground surface via a short-circuit connection.
The object of the present invention is to provide a two-dimensional radiator component with the property that it can produce linearly polarized and spatially wide sector radiation both in the azimuthal and in the elevation plane, as well as pronounced back-radiation attenuation and therefore useful radiation exclusively within a space hemisphere preferably in the spectral ranges of between 890 MHz and 960 MHz or 1710 MHz and 1890 MHz.
This object is achieved by the invention according to the distinguishing part of claim 1. Through the conductive connections of the two layers, a reduction in the overall size by approximately a factor of 2 is obtained, since xcex/2 waves can advantageously be received. By virtue of the fact that the distance between the rectilinear edge and the short-circuited border changes, it is possible to receive and transmit in a relatively broad spectral range. In this case, it is advantageous if the first layer is circular and the second layer is reduced compared with the first by a chord portion, the chord portion forming a rectilinear edge. In this case, the conductive connections on the border side remote from the rectilinear edge may be produced by means of connection elements in point form or strip form. It may, however, also be advantageous to configure the base surface first layer elliptically, in a triangular shape, square or hexagonally.
The oscillation conditions of the planar radiator can advantageously be implemented by simulation software to investigate field problems of radio frequency radiation. In this regard, it should be noted that, for each spectral range, a comprehensive set of different oscillation conditions need to be taken into consideration depending on the radiator characteristic. Since full calculation taking these boundary conditions into account is not possible, the person skilled in the art is inevitably led to simulation trials if he wants to configure the planar radiator according to the invention to his circumstances.
The oscillation conditions of the planar radiator can also advantageously be influenced using diaphragms produced in the second layer by holes in the conductive layer. In this regard, the diaphragms form implemented capacitors with distributed parameters, which in this form electrically extend the waveguide geometry or provide the possibility of geometrical miniaturization. The arrangement of the diaphragms is in this selected symmetrically, since the symmetry condition represents the prerequisite for preserving the preferential polarization of the electric field vector. In this case, by means of the diaphragm position, the possibility is provided of altering the oscillation direction of the field vectors, which are primarily affected by the diaphragms, and therefore the resultant field profiles which are created by superposition. The location where the diaphragms are introduced and, subordinately, the diaphragm contour determine the degree to which the conduction currents, as well as the associated electric and magnetic field components, are affected. To that extent, the position and contour of the diaphragms primarily dictate the raising or lowering of the capacitive and inductive components within the reactive component budget. Since the diaphragms which are introduced basically influence the complex waveguide properties, besides alteration of the spectral oscillation conditions, the possibility is thereby provided of affecting the spectral bandwidth of the type of oscillation excited. The surface of each diaphragm may in this case either be circular, elliptical, rectangular, square, triangular, hexagonal or irregular. The optimum shape of the diaphragms and their arrangement can in turn usually be established only empirically by simulation tests. The electromagnetically resonating oscillation arrangement is excited or fed by means of a coaxial waveguide, the inner conductor of the waveguide being conductively connected to the second layer and the outer conductor of the waveguide being conductively connected to the first layer, the inner conductor being arranged through a diaphragm within the first layer, axially symmetric to the diaphragm border and without electrical connection to the latter.
The way in which the two layers are in connection with one another along border remote from the rectilinear edge is freely selectable. It is thus possible to connect these two layers to one another by means of conductive pins. This is advantageous especially when no dielectric is arranged between the two layers, and the two layers are formed, for example, by copper plates. The conductive connection pins are then used, as it were, as spacers.
If a dielectric is arranged between the two layers, it may be used as a support for the two conductive layers, the conductive connection then being advantageously made outside the dielectric, to which end the dielectric may be coated in linear or two-dimensional form on its outer edge.
The shape of the border on which the two layers are conductively connected to one another, is in principle freely selectable, although care must be taken to comply with the oscillation conditions. If the border remote from the rectilinear edge or chord extends parallel to the chord, only a monochromatic frequency response can be obtained. It is therefore necessary to design this border edge non-parallel to the rectilinear edge or chord of the second layer if a frequency spectrum or band is desired.
The planar radiator according to the invention forms an optimum antenna component or replacement component for the external vehicle antenna with the possibility of being mounted inside the passenger compartment. The application field further relates to general indoor applications, in that the radiator component forms a component spatially remote from the terminal in question and can be mounted on the inside and in two-dimensional fashion on the relevent room glazing. It is also possible for the room glazing itself to serve as the dielectric support of the conducting two layers.
The radiator component or planar antenna according to the invention can advantageously be used in all cases in which the space lying behind the antenna aperture is to be kept free of radiation or at a low radiation level, and the exposure of the user to electromagnetic radiation is therefore to be minimized.
Further, the radiator component according to the invention forms a base module for short or medium range transmission systems for communication, sensor or safety applications.